Gaseous sulfur dioxide absorption system

ABSTRACT

A system for the absorption of SO2 from gases of combustion where the gases are passed in series through a plurality of direct contact zones. The absorption liquid is made up of a solution of magnesium and sulfur which is sprayed into the gas, and the makeup water in the solution is selectively added into the last stage zone for optimum SO2 absorption efficiency of the entire system.

United States Patent SYSTEM 2 Claims, 4 Drawing Figs.

U.S. Cl 23/2 R, 23/130, 23/178 S Int. Cl BOld 53/34 Field of Search .f.23/2, 178, 1788,130, 131

[56] References Cited UNITED STATES PATENTS 2,351,780 6/1944 Palmrose23/130 3,273,961 9/1966 Rogers et al. 23/178 X Primary Examiner- Earl C.Thomas Attorney-J. Maguire ABSTRACT: A system for the absorption of $0from gases of combustion where the gases are passed in series through aplurality of direct contact zones. The absorption liquid is made up of asolution of magnesium and sulfur which is sprayed into the gas, and themakeup water in the solution is selectively added into the last stagezone for optimum SO absorption efficiency of the entire system.

PATENTEDUCT 2619?! 3,615,165 SHEET 1 [IF 3 SOLUBILITY LIMIT AT 145 F 6F|G.2 a

DESIGN CONDITIONS A $02 ENTERING 024% VOL. WET

MAKEUPWATER FLOW. LB/T PERCENT 502 AS M050 0 0 2 3 4 INVENTOR.

PERCENT TOTA L 50 John L.Clemenf ATTORNEY PATENTEUUBT 26 ml SHEET 2 OF 3FIG.3

ACID CONCENTRATION l TOTAL 50 PATENTEnum 2s 19?! SHEET 3 BF 3 FIG.4

THIRD STAGE SOz ABSORPTION EFFICIENCY, /o

GASEOUS SULFUR DIOXIDE ABSORPTION SYSTEM The present invention relatesto the absorption of the S contained in combustion gases and moreparticularly to the absorption of gaseous SO, in an absorption liquidcontaining magnesium.

Magnesium base chemicals in liquid form have proven highly effective inremoving S0 gases from the products of combustion resulting from theincineration of residual liquors. When used in the chemical recoverysystem of a pulp and paper installation, both the magnesium compoundsand the S0 absorbed in the magnesium may be reused in the chemicalprocess. This procedure has been disclosed and claimed in U.S. Pats.Nos. 2,268,456 and 3,092,535, for example. However, the SO absorptionfrom flue gases may be applied to the products of combustion resultingfrom both the incineration of pulp residual liquors and the combustionof other sulfurcontaining fuels. When used in the latter case, theabsorbed S0 may be separately processed for reclamation of the sulfurcomponents in a useable sulfur form as for example S0 liquid, sulfuricacid or as elemental sulfur. In such a process, the magnesium compoundsmay be recycled and treated for reuse in the SO,' absorption system.Under such circumstances, it will be necessary to add magnesium to theabsorption system as a makeup for necessary losses of magnesiumcompounds. However, the magnesium makeup requirements can be a minimumand can be relatively inexpensive depending upon the source of themagnesium compounds.

Insofar as the S0 absorption is concerned, regardless of the source of$0 in the gases, the magnesium base absorbing liquid will be introducedin spray contact relationship to the S0 containing gases in, forexample, a series arrangement of venturi scrubbers. In each of theventuri scrubbers the S0 containing gases will be contacted by amagnesium-sulfur liquid spray where the active S0 absorbing ingredientwill be magnesium monosulfite. In each of the absorbing zones, theliquid and gases will be separated, the gases passed in series throughfurther absorbing zones in contact with recirculated, treated absorbingliquid. The general chemical procedure follows the following reactions:

If MgO is added to the aqueous lWgIHSQ-Q solution of fixed S0concentration, the MgO will react with Mg(HSO forming MgSO according toreaction (4). As a result, the Mg ion concentration in the acidincreases. When the amounts of MgO, S0 and H 0 are in solutionequilibrium, a saturated solution of Mg(l-ISO MgSO in water will beobtained. If the SO, concentration is increased, then reaction (6) willoccur, converting MgSO to Mg(I-IS0 and no precipitation of MgSO or MgOwill take place. Decreasing the dissolved S0 concentration to change theequilibrium proportion of MgO, S0 and H 0, the excess MgO willprecipitate out of the solution. Increasing the H 0 content will merelydilute the solution, whereas decreasing the H 0 content will change theequilibrium and tend to precipitate MgSO In the present invention it hasbeen found that most efficient absorption of $0 in the variousseries-arranged absorption zones can be obtained by careful regulationof the chemical consist of the individual spray liquids. As the S0 gasis passed in series through successive absorption zones the S0 contentof the gases will be progressively reduced. Thus, in each absorptionzone it will be desirable to carefully regulate both the acid strengthand the monosulfite content of the spray liquid to correspond with theS0 gas content for most efficient absorption. This can be accomplishedby regulation of the addition of magnesium hydroxide and water to eachof the zones. In the present invention, the product of the absorptionsystem is removed in liquid form from the liquid gas separating zone ofthe first 50 absorption zone. Thus, the total liquid added to the systemwill be commensurate with the liquid removed from the system.

Of the drawings:

FIG. 1 is a schematic flowsheet of an S0 absorption system constructedin accordance with the present invention; and

FIGS. 2, 3 and 4 are curve sheets showing the effect of the addition ofmakeup water to an $0 absorption system.

When used for the absorption of S0 from the products of combustion fromthe incineration of the residual liquor resulting from magnesium basepulping of cellulosic materials, the equipment arrangement will begenerally as taught in the U.S. Pat. No. 3,273,961. In such a system themagnesium and sulfur components in the residual liquor are recovered foruse in the cellulosic pulping process.

When used to absorb $0: from the gases resulting from the combustion ofother sulfur containing fuels, a magnesium base liquid may also be usedwith the resulting product liquid sub sequently treated to separate thesulfur and magnesium components. In such a system the sulfur may bereclaimed in concentrated form with the magnesium components recycled tothe absorption system. Regardless of the source of the S0 containinggases, i.e. whether from the combustion of residual magnesium basepulping liquor or other sulfur containing fuels, the absorption systemwill be substantially the same. In either case, the gases leaving thecombustion area are cooled by heat exchange to produce usable steam andto heat combustion air. Before passing through the S0 absorption zonesthe entrained solids in the combustion gases are largely removed and anydesirable further cooling of the gases is accomplished. Ordinarily, thegases passing through the S0 absorption zones will be cooled to the dewpoint temperature of about F. by spray contact with the absorbingliquid.

In the illustrated embodiment of the invention the numeral 10 representsthe combustion apparatus, gas-cooling arrangements and MgO dust removalequipment common in the art, with the cooled gases containing S0 and aresidue of M g0 dust passed through a gas duct 11 into the first stageor zone 12 of the S0 absorption system of the invention. It will beunderstood that the S0 absorption system of the present invention is notlimited to the particular furnace construction disclosed in the referredto patent, but is applicable to the absorption of S0 contained incarrier gas by a liquid containing magnesium where the most activeabsorbent is magnesium monosulfite.

The gases entering the first stage venturi scrubber 12 through the duct11 are contacted by a liquid injected into the gas by a multiplicity ofnozzles 13 positioned upstream, in a gas flow sense, of the convergingportion 14 of the venturi. The liquid spray includes recirculated liquidintroduced to the manifold 16 through pipe 17. In leaving the venturithroat 18 the gas and liquid mixture passes through a pressure regain,diverging section 20 to a tank 21 provided at the lower end of theventuri unit 12. The tank is constructed with an upper cylindricalportion 22 and a lower inverted frustoconical portion 23 with separationof entrained liquids and solids from the gases occurring in agas-turning space in the portion 22. A further cleaning effect may beobtained by the use of a pad (not shown) of compacted metal mesh or thelike positioned adjacent the gas outlet 24 from the tank. The separatedliquid accumulates in a pool in the portion 23 of the tank and iswithdrawn in part by a pump 25 for delivery to the pipe 27 and thence tomanifold 16. The quantity of liquid recirculated to the manifold 16 iscontrolled so that the liquor contacting the gas will be coordinatedwith the gas flow quantity introduced through the duct 11. A controlledflow of liquid is withdrawn from the tank 21 through pipe 29 fordisposal. Such liquid contains the absorbed SO, in the form of magnesiumbisulfite, with the liquid thereafter treated to form cooking acid inthe pulping process or converted to other useful forms, i.e. elementalsulfur, concentrated SO liquid or the like by known procedures.

The gases leaving the first stage venturi 12 pass through a duct 26connecting the outlet 24 in the upper end of the tank 2] with the inletside of the second stage venturi scrubber 27 wherein the gases are againintimately contacted by a spray liquid from males 28 positioned upstreamin a gas flow sense,

of the throat portion 30 of the venturi. As described in connection withthe venturi unit 12, the gases in turning towards the gas outlet duct 32of the tank 31 encourage separation of entrained liquid from the gaswith further liquid separation obtained by the use of a demister pad ofcompact wire mesh (not shown) positioned in the entrance of the gasoutlet duct 32 from the tank 31. The separated liquid accumulates in thelower inverted frustoconical portion 33 of the tank 31 from which liquidis withdrawn by a pump 34 through a pipe system 35 interconnecting thetank 31 and the spray nozzles 28 for recirculation of liquid intocontact with the gases passing through the venturi 30.

The gases pass through the duct 32 to the third stage venturi scrubber36, where they are again subjected to spray contact with liquidintroduced through nozzles 37 upstream of the throat portion 38 of theventuri. The unit 36 is similar to both of units 12 and 27 in that thegas and liquid contacted in the venturi discharges into a lower tank 40where the gas and liquid is separated by change of flow direction withthe gas discharging through duct 41 and the liquid accumulating in theinverted frustoconical portion 42. A pump 43 withdraws liquid from theportion 42 for delivery to the spray nozzles 37 through a pipe system44.

The'weight of liquid recirculated to each of the nozzles 13, 28 and 37from the pumps 25, 34 and 43, respectively, will have a ratio of theorder of or to 1 relative to the weight of gas passing through theabsorption zones so as to attain desirable gas and liquid contact.

As shown, a pipe 45 connects the tank 40 of venturi scrubber 36 with thebottom portion 33 of venturi scrubber 27, while in a similar manner,tank 31 of venturi scrubber 27 is connected by a pipe 46 with the bottomportion 23 of venturi 12. The pipes 45 and 46 pass a controlled flow ofliquid from stage to stage of the absorption system in a directioncountercurrent to the gas flow through the absorption stages. Or-

- dinarily, the rate of flow through the pipes 45 and 46 is uniform tomaintain a substantially constant volume of liquid in the bottomportions of the units 36, 27, and 12.

Makeup water is provided to the absorption system via a pipe 47, whichis provided with valved branch pipes 48, 50, and 51 connected with thepiping systems leading to the inlet side of the recirculating pumps 43,34, and 25 respectively. The magnesium hydroxide slurry is prepared byslaking magnesium oxide (not shown) and pumping it through a pipe 52which is arranged for closed-circuit flow and is provided with valvedoff-take branch pipes 53, 54, and 55 connected with the inlet of therecirculating pumps 43, 34, and 25 respectively. The flow of themagnesium oxide to each of the pumps is regulated to provide as large aquantity of magnesium monosulfite in the liquid sprays as possible,consistent with the solution solubility limits in the particular venturiabsorption unit.

In the operation of the SO, absorption system shown in FIG. 1, the fluegases entering through the duct 11 may have an S0, content which isdependent upon the fuel burned. When burning residual pulp liquor fromthe magnesium base pulping process the SO, content in the gases may beof the order of 1.0 percent by volume on a wet gas basis. When burningother fuels the SO, content may be lower or greater depending upon theoriginal sulfur content, but may also be of the same general value. Forpurposes of illustrating the invention, it will be assumed the SO,content of the gases to be treated will be 1 percent (volume of wetgas).

it will also be understood the number of serially connected absorptionstages may be either more or less than that shown, depending upon theSO, content of the entering gas, and the degree of SO, removal desired.With an S0, content of 1 percent and a three-stage arrangement as shown,an excess of 98 percent of the gaseous SO, will be absorbed when theapparatus is operated in accordance with the present invention.

The efficiency of the SO, absorption system is considerably influencedby the strength of the product acid withdrawn from the first stageabsorption unit 12 through the line 29. The strength of the acidwithdrawn is largely dependent upon the end use thereof. While in an S0,absorption process connected with an ordinary fuel-fired system theproduct acid strength may be regulated for highest SO, recovery, in acellu losic pulping process the strength of the product acid must beregulated to be compatible with the composition of the pulpcookingliquor. For example, a pulp mill utilizing an acid sulfite cookingprocess might require a total 80, requirement. of 4 percent in theproduct acid discharged through line 29 of the absorption system. Undersuch conditions with a specific gas flow through the absorption systemmakeup water flow to the system to compensate for acid withdrawal willbe 9,200 pounds per ton of pulp produced per day. In a pulp millutilizing a product acid of 6.24 percent total SO, and 3.52 percentcombined SO, the makeup water flow to the system under otherwisecomparable conditions would be 5,200 pounds of pulp produced per day.

FIG. 2 is a curve drawn to illustrate the effect of makeup water flow tothe last stage (36 in FIG. I) of the absorption system when the productacid discharged through line 29 has a total SO, concentration of 4percent. In this figure, the or dinate indicates the percent SO, presentin the liquid as magnesium monosulfite m so and the abscissa indicatesthe percent total SO, in the liquid. The solid curve indicates thesolubility limit of the MgSO, in the liquid at a typical temperature ofF. The makeup water flow rate to the last stage absorption unitestablishes the percent concentration in the liquid of magnesiummonosulfite and total S0,, and the curve indicates several makeup waterflow rates to show the relationship between such rates and its effect onthe concentration.

FIG. 3 is a curve drawn to illustrate the effect of acid concentrationin the third SO, absorption zone on the efi'iciency of S0 removal fromthe gas stream in that respective stage, recognizing that theconcentration in the stage is directly related to the rate of additionof makeup water to the last absorption unit. The abscissa shows the SO,absorption efficiency in percent and the ordinate shows theconcentration of total SO: in the liquid contained within the absorptionzone. The relationship between absorption efficiency of SO, gas (FIG. 3)is established by the makeup water flow rate as indicated from FIG. 2.

For example, using FIGS. 2 and 3, it will be noted that for a 4-percenttotal SO, in the absorbing liquid discharged from the absorption systemto line 29, the makeup water flow to the system would be 9,200 poundsper ton of pulp per day. If all of this water was delivered to the laststage absorption unit, the acid concentration in the absorption zonewould be 0.62 percent total SO, resulting in an absorption efficiency of73 percent according to FIG. 3.

According to this invention, the makeup waterwould be divided with thelast stage unit receiving 3,600 to 5,200 pounds of water, and theremaining makeup water delivered to the second stage unit (27 in FIG.1). Under these conditions as shown in FIG. 3, the absorption efficiencyof the third stage unit will be improved to 91 to 93 percent. Theremaining makeup water diverted to the second stage absorption unit willhave an insignificant effect on efiiciency of the first and second stageunit for the reason that the acid concentration in the first and secondstage units will be no different than if the total makeup water rate of9,200 pounds were added to the third stage unit. The difference in S0,absorbed in the third stage unit over the range of operation possiblebetween makeup water flow rates to that stage of 3,600 to 9,200 poundsof water is small compared to the total amount of SO, absorbed in thesecond and third stage units, and, therefore, has an insignificanteffect on the concentration of the acid in the second stage unit.

FIG. 4 is a curve drawn to illustrate the effect of the S0, absorptionefficiency of the third stage absorption unit on the total overallefficiency of the system. The efficiency of the last stage unit has adirect and proportional effect on the overall efficiency.

In operating the absorption system according to the invention ashereinbefore described will increase the overall efficiency of thesystem 4 to 6 percent without any increase in capital or operatingcosts. Such an increase in absorption efficiency will reduce the loss ofcostly sulfur and substantially completely eliminate the nuisance of 80,gas discharge to the atmosphere.

What is claimed is:

l. The method of absorbing gaseous S0 present in flue gases whichcomprises passing said gases in series through a plurality of SO,absorption zones in direct contact with an atomized liquid containing asolution of magnesium com-' pounds including magnesium monosulfite,separating gas and liquid from each absorption zone, reinjecting a majorportion of said separated liquid into the corresponding absorption zone,passing a minor portion of said separated liquid at a controlled ratefrom one absorption zone to another absorption zone countercurrent tothe direction of gas flow through said zones, withdrawing separatedliquid from the first of said absorption zones in a gas flow sense,adding a controlled quantity of magnesium slurry to each of said zonesto regulate the monosulfite content therein, and adding a divided flowof makeup water to said absorption liquid in said absorption zones foroptimum SO, absorption therein and to compensate for the separatedliquid withdrawn from said first absorption zone.

2. The method of absorbing gaseous SO, according to claim 1 wherein atleast three of said absorption zones are arranged for serial flow ofcontaining gases therethrough, and said makeup water flow isproportioned between the last and intermediate absorption zones in a gasflow sense to increase the gaseous SO, absorption efficiency of saidlast zone.

2. The method of absorbing gaseous SO2 according to claim 1 wherein atleast three of said absorption zones are arranged for serial flow of SO2containing gases therethrough, and said makeup water flow isproportioned between the last and intermediate absorption zones in a gasflow sense to increase the gaseous SO2 absorption efficiency of saidlast zone.